This invention is in the field of audio amplifiers, and is more specifically directed to pre-amplifier circuits for pulse-width modulated class D audio power amplifiers.
As is fundamental in the art, electronic amplifier circuits are often classified in various “classes”. For example, the output drive transistors of class A amplifier circuits conduct DC current even with no audio signal, and the entire output voltage swing is of a single polarity. A class B amplifier, on the other hand, typically includes complementary output drive transistors, driving an output voltage swing including both positive and negative polarity excursions. Class B amplifiers are necessarily more efficient, because both complementary output drive transistors are never on at the same time. Class AB amplifiers maintain a small bias current through complementary output drive transistors, so that the output voltage swing is centered slightly above (or below) ground voltage. While the non-zero bias current renders class AB amplifiers theoretically less efficient than class B amplifiers, class AB amplifiers present avoid the crossover distortion of class B amplifiers.
In recent years, digital signal processing techniques have become prevalent in many electronic systems. The fidelity provided by digital techniques has increased dramatically with the switching speed of digital circuits. In audio applications, the switching rates of modern digital signal processing are sufficiently fast that digital techniques have become accepted for audio electronic applications, even by many of the fussiest “audiophiles”.
Digital techniques for audio signal processing now extend to the driving of the audio output amplifiers. A new class of amplifier circuits has now become popular in many audio applications, namely “class D” amplifiers. Class D amplifiers drive a complementary output signal that is digital in nature, with the output voltage swinging fully from “rail-to-rail” at a duty cycle that varies with the audio information. Complementary metal-oxide-semiconductor (CMOS) output drive transistors are thus suitable for class D amplifiers, as such devices are capable of high, full-rail, switching rates such as desired for digital applications. As known in the art, CMOS drivers conduct extremely low DC current, and their resulting efficiency is especially beneficial in portable and automotive audio applications, and also small form factor systems such as flat-panel LCD or plasma televisions. In addition, the ability to realize the audio output amplifier in CMOS enables integration of an audio output amplifier with other circuitry in the audio system, further improving efficiency and also reducing manufacturing cost of the system. This integration also provides performance benefits resulting from close device matching between the output devices and the upstream circuits, and from reduced signal attenuation.
In recent years, single-hip audio amplifier systems now integrate the audio output amplifier with conventional pre-amplifier circuits. Such integration not only provides the benefits mentioned above, but also facilitates closed-loop circuit techniques for reducing harmonic distortion. Integration of the audio output amplifier with the preamplifier also enables the system designer to further miniaturize the end system.
FIG. 1 illustrates a conventional class D audio output amplifier and preamplifier architecture, such as may be realized in a single integrated circuit. Input voltage Vin is a differential signal, such as may be generated by an upstream codec or other signal processing function. DC blocking capacitors 3 capacitively couple the differential input signal Vin to the inputs of differential preamplifier 5. Preamplifier 5, in this conventional architecture, is realized by relatively low-voltage devices, at least relative to the class D power amplifier stage 11.
Power amplifier stage 11 is a conventional class D power amplifier, applying a fixed gain (e.g., a gain of five). In this conventional example, the differential output of preamplifier 5 is coupled, through resistors Ri, to differential inputs of integrating amplifier 7. The differential output of integrating amplifier 7 is applied to the differential input of pulse-width modulating (PWM) amplifier 9, which drives its differential output at a PWM output voltage Vout, in class D fashion. Feedback resistors Rf couple the output of PWM amplifier 9 back to the differential input to integrating amplifier 7.
While this conventional preamplifier and amplifier arrangement of FIG. 1 has been well-accepted in the industry, it inherently boosts any noise and offset voltage that appears at the output of preamplifier 5. In this example:
                              V          out                =                              (                                          V                n                            +                              V                os                                      )                    ·                      (                          1              +                                                R                  f                                                  R                  i                                                      )                                              (        1        )            where Vn and Vos are the noise and offset voltages, respectively, at the input to integrating amplifier 7. In this case, if the gain of power amplifier stage 11 is five
      (                            R          f                          R          i                    =      5        )    ,then the noise and offset voltages are boosted by a factor of six at output voltage Vout. An offset voltage of 5 mV thus results in an output offset voltage of 30 mV. This offset boost results in increased bias current at the audio output, a higher noise floor, and louder “popping” as the amplifier is turned on.
One could reduce the effect of the noise and offset voltage by “chopping” the integrating amplifier, so that the boosting effects are moved out of the audio frequency band. However, this is impractical due to the very large slew rate required to charge and discharge these large capacitors in handling high amplitude voltage swings.
By way of further background, a circuit referred to as the operational transconductance amplifier (OTA) is well known in the art. OTAs are operational amplifiers that receive an input voltage and produce an output current, thus serving as a voltage-controlled current source. The transconductance parameter can be controlled by the bias current in the differential input pair or by a resistor (or both), and very high input and output impedances are presented by the OTA. An example of a feedback OTA circuit is shown in Pierce, et al., Applied Electronics (Bell & Howell, 1991). p. 448.
Conventional OTA architectures as applied to audio preamplification have been proposed. It has been observed, in connection with this invention, that many of these OTA approaches contribute to significant DC offset voltages at the output of the amplifier. For example, the TDA8922 class D power amplifier available from Philips Semiconductors specifies a maximum DC output offset voltage of 150 mV. This high output offset voltage is believed, in connection with this invention, to be for a likely cause of speaker “popping” on power-up and of increased quiescent current consumption.